It is known to form sapphire watch crystals which are highly scratch resistant. These crystals are generally manufactured by placing a rotating grinding wheel in contact against the surface of a drum carrying several crystals. The resulting grinding operation enables a cylindrical or spherical crystal to be formed. However, it becomes necessary to form asymmetrical crystals to be fitted, for example, to timepiece displays which are not centred with respect to the timepiece case, which is not possible using current series manufacturing techniques.